The present oil extracting rod ground screw pump driving system employs common asynchronous motor through belt transmission to drive gear box, and the gear box then drives the polished rod of the screw pump to make rotary motion. The dynamic system for such a screw pump has the following disadvantages:
1. The transmitted power is limited. When the screw pump requires bigger torque (for example, a torque bigger than 1500 Nm) or the system transmitted power is bigger (for example, bigger than 10 kW), belt easily slips and be damaged, teeth of the gear box are easily broken and damage, therefore it's difficult to satisfy the requirement.
2. A large amount of mechanical maintenance and high operation cost are needed. Rotary sealing parts must be installed within the gear box because there is lubricating oil in the gear box. In practical operation, problem of lubricating oil leakage always happens in said gear box. Moreover, the reliability of belt transmission is poor, which leads to the large amount of mechanical maintenance and high operation cost during practical operation.
3. The efficiency is low. In general, for the convenience of starting, the equipped motor is always chosen to have 3 times the power as that of the practical operation power, which leads to the operation of asynchronous motor under low load rate, thus the efficiency is rather low and generally is about 75%.
4. The regulation of parameter is not convenient. When the speed need to be re-regulated in case that the downhole working condition varies, generally, the belt pulley need to be changed or variable pole adjustable speed motor need to be employed. The changing of belt pulley is very troublesome, while just one to two gears can be used through the speed adjusting of the variable pole adjustable speed motor, which make it really not convenient in practical usage.
5. The noise is great. The noise during the operation of mechanical speed reducing devices and high speed asynchronous motor is great and thus it is not suitable to installed in residential districts.
6. The occupied are for the equipment is large. Since the belt pulley system requires a large occupied area, said equipment is not suitable for the situation that required area is small, such as the situations of offshore oil-production platform or cluster well etc.
A low speed motor direct drive screw pump device has been provided in the disclosed invention patent application 200510042122.8, said device consists of polished rod, shaft coupling, low speed motor, motor controller, fixing seat, screw pump, in which the motor shaft of said low speed motor is set hollow and through the polished rod, it was fixed with polished rod as one through shaft coupling, the polished rod passes through the central hole of the fixed seat and connects to the shaft of screw pump downhole, low speed motor is fixed onto the fixed seat. The motor controller is installed within the terminal box of low speed motor, matching controller for the low speed motor is employed. Such a motor directly driving screw pump cancelled the usage of a belt transmission and a speed change gear device, motor shaft is connected to the pump shaft of screw pump through polished rod, the reactive power loss is decreased, the system efficiency and reliability of screw pump is increased, the oil extracting cost is reduced, at the same time, the volume and weight of the screw pump is reduced.
However, said device does not realize the selectively precise regulation of any value of motor speed from zero to maximum by controller, and said device has the disadvantage of inconvenience of speed regulation, moreover, the efficiency of the system needs further increase.